Overview
This lecture explains the distinction between polar and non-polar molecules, the role of electronegativity and molecular geometry in determining polarity, and how these concepts relate to solubility and properties of water.
Molecular Polarity: Basics
- Molecules are classified as polar or non-polar based on charge symmetry.
- Polar molecules have uneven electron distribution and charge separation; non-polar molecules are symmetric in both geometry and charge.
Determinants of Polarity
- A molecule must contain at least two different elements to be polar, with an electronegativity difference of 0.5 or greater.
- Electronegativity measures an atom's tendency to attract electrons; it increases across a period and decreases down a group.
- For polarity, both charge asymmetry (from different electronegativities) and geometric asymmetry are required.
Molecular Examples and Geometry
- CO₂ and CH₄ have polar bonds but are non-polar molecules due to symmetric geometry, which cancels out the dipole.
- A dipole moment exists when there is both a polar bond and asymmetric geometry, resulting in separated positive and negative areas.
Notation and Visualization
- Dipole moments are shown by arrows pointing toward the negative side, with δ⁺ and δ⁻ indicating partial charges on atoms.
Intermolecular Forces and Solubility
- Like dissolves like: polar liquids dissolve polar or ionic solids; non-polar liquids do not mix with polar ones.
- Water's cohesion is due to strong intermolecular (hydrogen) bonds, leading to surface tension and organized molecular alignment.
Hydrogen Bonding and Water's Unique Properties
- Water molecules form hydrogen bonds due to partial negative oxygen and partial positive hydrogen atoms.
- In ice, all O and H atoms are hydrogen-bonded, making ice less dense than liquid water, causing it to float.
- Water has a high specific heat capacity due to energy absorbed or released in hydrogen bond changes, stabilizing Earth's temperature.
Polar, Non-polar, and Hybrid Molecules
- Surfactants (like in soap) and cell membrane fatty acids have both polar (hydrophilic) and non-polar (hydrophobic) regions.
- These hybrid molecules allow for interactions with both water and non-polar substances.
Key Terms & Definitions
- Polar molecule — has unequal electron distribution, resulting in partial positive and negative sides.
- Non-polar molecule — has equal electron distribution, resulting in no overall charge separation.
- Electronegativity — the tendency of an atom to attract electrons in a bond.
- Dipole moment — a measure of charge separation within a molecule.
- Hydrogen bond — a weak bond between a partially positive hydrogen and a partially negative atom (like oxygen).
- Specific heat capacity — the amount of heat required to change a substance's temperature.
Action Items / Next Steps
- Review periodic trends in electronegativity.
- Practice identifying polar and non-polar molecules using Lewis structures and geometry.
- Read about hydrogen bonding and its effects on physical properties of water.